Like I said poor quality control. It is difficult with a thin aluminum casting to keep it flat. When it is machined it heats up expands then cools contracts and deforms. I guess some people are lucky and get a flat one.
I believe you are right about poor QC. But even in the old analog only days, this aluminum metallurgy technology has been around for decades. The master lathe craftsmen would have never let that platter leave the shop. They have been crafting high quality high tolerance aluminum parts well before NASA in the 60’s!
This is what happens when trying to save money from doing everything in house you outsource all you can to the lowest bidder. Bummer. I’m glad I still have my minty Pioneer PL-630 table, and won’t be selling it. Thanks OP.